The present invention generally relates to the field of wireless communication devices, and more particularly to devices adapted for communication on one or more communication bands and an auxiliary band.
Wireless communications systems generally have base stations and antennas which communicate with mobile wireless devices. These wireless devices may accommodate voice communication as well as data communication. For example, the wireless devices may be mobile phones, personal data assistants, or laptop computers. Since they are portable, the wireless devices are usually powered by a battery, and need to be sized for convenient use. Most commercial wireless communication systems comply with some international standard to assure compatibility between the base station and the mobile devices. However, since there are several communication standards, mobile devices can only communicate in a compatible communication system. Since this has been found to be too limiting, mobile wireless devices are often adapted to operate in more than one communication system.
Wireless communication systems are generally arranged to operate in particular frequency range or communication band. For example, one communication system may operate in the PCS band, which operates in frequency band at approximately 1900 MHz. Another communication system may operate in the cellular band, which operates in frequency band at approximately 800 MHz. It has been found to be desirable to have mobile wireless devices capable of operating at multiple communication bands, and thereby able to operate on multiple communication systems. For example, it has been found particularly useful to have wireless mobile devices capable of operating on both the PCS and cellular systems. It will be appreciated that other frequency bands and communications standards may be used.
It has also been found desirable that a mobile wireless device be constructed to receive an auxiliary signal. A particularly useful auxiliary signal is the position location information signal provided by a GPS satellite system. In the GPS system, several satellites transmit a location beacon at about 1575 MHz that may be received by a GPS receiver. The GPS signal contains timing and location information that may be used to determine the location of the GPS receiver. In this way, a mobile wireless device receiving the GPS signal may provide location information for emergency personnel or other applications. However, adding multiple band capability and adding auxiliary signal capability to wireless mobile devices has made these devices more complex and more costly to build.
Even though mobile wireless devices have become more complex, market pressures demand higher performance and lower costs. In this regard, the manufacturing of mobile devices requires trade-offs between loss, noise, and price. For example, a lower loss device may require the use of more costly components. Such a lower loss device would have a longer battery life in a portable device, which is highly desirable in the mobile wireless market. In another example, it may be desirable to make a less costly phone by using fewer or lower quality components. However, such a construction may lead to additional noise in the receive or transmit circuitry, thereby reducing the sensitivity of the phone. Such a reduced sensitivity may lead to poor reception, poor signal quality, dropped calls, or other undesirable performance characteristics. With the increasing push for additional features in the mobile wireless device, there is great market pressure to reduce cost while improving noise and loss characteristics.
A wireless device constructed to operate on multiple communication bands generally has some form of band selection circuitry. In this way the device may communicate on one band, and when conditions warrant a change, switch to communicate on the other band. For example, a mobile phone may operate on the PCS band, but when the phone is moved to a location without a PCS service, then the phone may select to operate on the cellular band, if present. Of course, it would be desirable to allow the mobile phone to receive a GPS signal irrespective of which band is used for voice communication. In this regard, the band selection circuitry should be able to discriminate between at least two communication systems, but yet be able to receive an auxiliary GPS signal, if present. For example, a wireless device may need to selectively operate on the PCS communication system or the cellular communication system while receiving GPS signal information. In arranging band selection circuitry, it is known to use either RF switches or triplexers. As will be discussed more fully below, both RF switches and triplexers each have deficiencies.
Band selection circuitry using RF switches has been disclosed. RF switches have relatively low loss of between about 0.2 and 0.4 db, but the RF switch is an active device, and therefore is likely to add noise in both the transmit and the receive path. In particular, an RF switch is prone to add intermodulation and IP3 noise to the communication signal, which is known to substantially degrade device performance. In band select circuitry using an RF switch, control circuitry sets the RF switch to direct the communication path to a GPS filter and low noise amplifier, or to a diplexer. More particularly, when the mobile device desires position information, the control circuitry sets the RF switch so the antenna couples with the GPS filter and amplifier. When the mobile device needs to communicate on one of the two available communication bands, then the control circuitry couples the antenna with the diplexer. The diplexer in turn couples to a pair of duplexers, with each duplexer constructed to provide transmit and receive paths for one of the communication bands. For example, one of the duplexers would be constructed to provide transmit and receive paths for the PCS communication band while the other duplexer would provide transmit and receive paths for the cellular communication band. Unfortunately, the RF switch continuously adds noise into the communication paths, and also contributes to increased insertion loss. Further, RF switches are a relatively costly electronic component.
In designs where cost is a major consideration, the band selection circuitry may use a triplexer instead of an RF switch. Unfortunately, some common triplexers are inherently lossy and therefore may contribute to a higher overall insertion loss for the wireless mobile device. Such a high insertion loss may lead to shorter battery life and degraded signal quality for the mobile device. It may be possible to build triplexers with better loss characteristics, but these devices are often constructed to be relatively large and therefore consume valuable and limited circuit area in a wireless device. However, since the triplexer is a passive device, it has better noise characteristics than the RF switch. In use, a band selection circuit using a triplexer couples to an antenna, and separates the communication signal into three bands. For example, the first band may be the cellular band, a second band may be the PCS band, and the third band may be the GPS band. The triplexer couples to two duplexers, with each duplexer constructed to provide a receive and transmit path for its associated band. For example, a cellular duplexer connects to the cellular output of the triplexer, and a PCS duplexer connects to the PCS output of the triplexer. A filter couples to the GPS output of the triplexer and passes a low-level GPS signal, which then is amplified in a low noise amplifier. Often, the GPS filter is a SAW (surface acoustic wave) filter.
In another known example of band selection circuitry, the antenna couples to a standard diplexer where a cellular communication path and a PCS communication path are established. The diplexer couples to a cellular duplexer and to a PCS duplexer for providing transmit and receive paths for each band. A separate GPS circuit also couples to the antenna for detecting the GPS signal. The GPS path is tuned so that ideally it appears as an open circuit to cellular communication signals or PCS communication signals. However, such tuning is not precise and typically results in leakage current, which increases insertion loss for the overall device and also degrades the quality of the cellular or PCS signal. Further such an implementation also requires additional components and additional circuit space for detecting the GPS signal.
Therefore, there exists a need for band selection circuitry that provides low loss, low noise injection, compact design, and cost effective implementation.